1. Field of the Invention
This invention relates to converting heavy hydrocarbon oils such as topped crudes and heavy residual oils into lower boiling fractions. The invention relates to converting heavy hydrocarbons containing concentrations of Conradson carbon coke precursors and metal contaminants to form gasoline and other liquid hydrocarbon fuels and effect temperature restrained regeneration of catalyst particles thus used.
2. Description of the Prior Art
In general, gasoline and other liquid hydrocarbon fuels boil in the range of about 38.degree. C. to about 343.degree. C. (100.degree. to about 650.degree. F.). However, the crude oil from which these fuels are made contains a diverse mixture of hydrocarbons and other compounds which vary widely in molecular weight and therefore boil over a wide range. For example, crude oils are known in which 30 to 60% or more of the toal volume of oil is composed of compounds boiling at temperatures above 343.degree. C. (650.degree. F.). Among these are crudes in which about 10% to about 30% or more of the total volume consists of polycyclic compounds so heavy in molecular weight that they boil above 552.degree. C. (1025.degree. F.) or at least will not boil below 552.degree. C. (1025.degree. F.) at atmospheric pressure.
Because these relatively abundant high boiling components of crude oil are unsuitable for inclusion in gasoline and other liquid hydrocarbon fuels, the petroleum refining industry has developed processes for cracking or breaking the molecules of the high molecular weight, high boiling compounds into smaller molecules which do boil over an appropriate lower boiling range. The cracking process which is most widely used for this purpose is known as fluid catalyst cracking (FCC). Although the FCC process has reached a highly advanced state, and many modified forms and variations have been developed, their unifying factor is that a vaporized hydrocarbon feedstock is caused to crack at an elevated temperature in contact with a cracking catalyst that is suspended in the feedstock vapors. Upon attainment of the desired degree of molecular weight and boiling point reduction the catalyst is separated from the desired products.
Crude oil in the natural state contains a variety of materials which tend to have quite troublesome effects on FCC processes. Among these troublesome materials are coke precursors (such as asphaltenes, polynuclear aromatics, etc.), heavy metals (such as nickel, vanadium, iron, copper, etc.), lighter metals (such as sodium, potassium, etc.,), sulfur, nitrogen and others. Certain of these, such as the lighter metals, can be removed by desalting operations, which are part of the normal procedure for pretreating crude oil for fluid catalytic cracking. Other materials, such as coke precursors, asphaltenes, porphyrins and the like, tend to break down into coke during the cracking operation, which coke deposits on the catalyst, impairing contact between the hydrocarbon feedstock and the catalyst, and generally reducing its potency or activity level. The heavy metals transfer almost quantitatively from the feedstock to the catalyst surface.
If the catalyst is reused again and again for processing additional feedstock, which is usually the case, the heavy metals can accumulate on the catalyst to the point that they unfavorably alter the composition of the catalyst and/or its catalytic effect upon the feedstock. For example, vanadium tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles sufficiently so that they begin to sinter and become ineffective cracking catalysts. Accumulations of vanadium and other heavy metals, especially nickel, are considered "poison" to the catalyst. They tend in varying degrees to promote excessive dehydrogenation and aromatic condensation, resulting in excessive production of carbon and gases with consequent impairment of liquid fuel yield. A crude oil and residual fractions of crude oil or other heavy oil sources that are particularly abundant in these metal contaminants exhibit similar behavior. Such heavy oil fractions which comprise relatively large quantities of coke precursors, asphaltenes and porphyrins are referred to as metallo-organic or carbo-metallic compound containing oils and represent a particular challenge for upgrading by the petroleum refiner.
In general, the coke-forming tendency or coke precursor content of an oil fraction can be ascertained by determining the weight percent of carbon remaining after a sample of that oil has been pyrolyzed. In conventional FCC practice, Conradson carbon values on the order of about 0.05 to about 1.0 are regarded as indicative of acceptable feed. The present invention is particularly concerned with the conversion of petroleum hydrocarbon feedstocks and residual portions thereof which provide greater than 1, up to about 12 or 14 Conradson carbon values and thus exhibit substantially greater potential for coke formation than lower boiling gas oil feeds.
According to conventional FCC practice, the heavy metal content of feedstock for FCC processing is controlled at a relatively low level, e.g. about 0.25 ppm Nickel Equivalents or less. The present invention is concerned with the processing of feedstocks containing metals substantially in excess of this and which therefore have a significantly greater potential for accumulating on and poisoning catalyst.
In the conventional FCC practice, in which a circulating inventory of catalyst is used agains and again in the processing of fresh feed, with periodic or continuing minor addition and withdrawal of fresh and spent catalyst, the metal content of the catalyst is maintained at a level which may, for example, be in the range of about 200 to about 600 ppm Nickel Equivalents. This process of the present invention is concerned with the use of catalyst having a substantial metals content in the range of 6,000 to 12,000 or more ppm of Ni+V and which therefore has a much greater than normal tendency to promote dehydrogenation, aromatic condensation, gas production or coke formation. Such high metals accumulation is normally regarded as quite undesirable in FCC gas oil processing.
There has been a long standing interest in the conversion of carbo-metallic oils into gasoline and other liquid fuels. Several proposals involve treating the heavy oil feed to remove the metal therefrom prior to cracking, such as by hydrotreating, solvent extraction, decarbonizing and demetallizing with relatively inert solids, complexing with Friedel-Crafts catalysts and combinations thereof, but these techniques have been criticized as unjustified economically. Another proposal employs a combination of cracking process having "dirty oil" and "clean oil" units. Still another proposal blends residual oil with gas oil and controls the quantity of residual oil in the mixture in relation to the equilibrium flash vaporization temperature at the bottom of a riser hydrocarbon conversion zone employed in the process. Still another proposal subjects the feed to a mild preliminary hydrocracking or hydrotreating operation before it is introduced into the cracking unit. It has also been suggested to contact a carbo-metallic containing oil feed such as residual or reduced crude oils with hot taconite pellets to produce gasoline. This is a small sampling of the many proposals which have appeared in the patent literature and technical reports.
It has been possible heretofore to largely avoid the problems of coke precursors and heavy metals by sacrificing the liquid fuel yield which would be potentially available from the highest boiling fractions. More particularly, a more conventional gas oil FCC practice has employed as feedstock that fraction of crude oil which boils in the range of at about 538.degree. C. (650.degree. F. to about 1000.degree. F.). Such fractions are relatively free of Conradson carbon coke precursors and heavy metal contamination. Such feedstock, known as "vacuum gas oil" (VGO), is generally prepared from crude oil by distilling off the fractions boiling below about 343.degree. C. (650.degree. F.) at atmospheric pressure and then separating the 343.degree. C. (650.degree. F.) plus fraction by vacuum distillation from the heavier resid fraction as vacuum gas oil boiling between about 343.degree. C. up to about 482.degree. C. or 552.degree. C. (650.degree. F. up to about 900.degree. F. or 1025.degree. F.).
The vacuum gas oil plus atmospheric gas oils is used as feedstock in conventional FCC processing to particularly produce high yields of gasoline. The heavier resid fraction of vacuum distillation is normally employed for a variety of other purposes, such as for instance the production of asphalt, #6 fuel oil, or marine Bunker C fuel oil. The present invention is concerned with effecting the simultaneous cracking of these heavier oil fractions containing substantial quantities of both coke precursors and heavy metals contaminants and possibly other troublesome components, in conjunction with converting the lighter gas oil fractions to desired gasoline product thereby increasing the overall yield of gasoline and other hydrocarbon liquid fuels from a given quantity of crude oil.
The oil feeds capable of being cracked by the method of this invention are carbo-metallic oils of which at least about 70 percent thereof boils above 343.degree. C. (650.degree. F.) and contains a carbon residue on pyrolysis of at least about 1 and at least about 4 parts per million of nickel equivalents of heavy metals. Examples of these oil feeds are crude oils, topped crudes, residual or reduced crudes, residua, and extracts from solvent deasphalting.
The unusually large amount of coke which deposits on the catalyst in carbo-metallic oil processing presents critical problems, the primary problem arising from the fact that the reactions in the regenerator which convert coke to water, carbon monoxide and carbon dioxide are highly exothermic. Using a carbo-metallic feed with its unusually high content of coke precursors as compared to gas oil FCC feeds, can substantally increase the amount of coke to be burned in the regenerator and thus the regeneration temperatures can become excessive in the absence of appropriate control if there is thorough burning of deposited coke. Excessive regeneration temperatures can permanently deactivate the catalyst and/or damage the regenerating equipment.
The heat of combustion of coke depends in part upon the concentration of hydrogen in the coke or carbonaceous deposit and the ratio of CO.sub.2 to CO obtained in the products of combustion. Carbon produces 13,910 BTU per pound of heat when burned to CO.sub.2 and only 3,962 BTU per pound when burned to CO. Hydrogen produces 61,485 BTU per pound of heat when burned to H.sub.2 O. The heats of combustion of coke for three representative levels of hydrogen and four different ratios of CO.sub.2 /CO are given in the following table:
TABLE 1 ______________________________________ Heat of Combustion BTU/lb Coke Percent Hydrogen CO.sub.2 /CO Ratio 2 4 6 ______________________________________ 0.5 8,362 9,472 10,582 1.0 11,472 12,083 3.0 14,446 4.0 12,912 14,894 ______________________________________
The problems encountered in regenerating catalysts coated with a high concentration of hydrocarbonaceous material may be aggrevated when catalysts of the crystalline zeolite or molecular sieve type are used. These catalysts, which comprise crystalline aluminosilicates made up of tetra-coordinated aluminum atoms associated through oxygen atoms with silicon atoms in the crystalline structure, are susceptible to loss of cracking activity upon extended exposure to high temperatures particularly in the presence of steam.
Various methods have been suggested and used to control the temperature in the regeneration zone including cooling by heat exchangers external to the regenerator (see U.S. Pat. No. 2,394,710), cooling by injection steam or water into the dense catalyst bed or an upper dilute phase thereabove in a regenerator (see U.S. Pat. Nos. 3,303,123 and 3,909,392), and controlling the amount of oxygen preent (see U.S. Pat. No. 3,161,583).